archiving disabled accounts

i need a suggestion
i would archive inactive accounts; i cannot delete them without loosing their emails, i cannot disable them without wasting license slots.
One solution can be to setup an opensource zimbra and put there these older accounts.
Of course the domains must be the same in the 2 servers, so i think to put OS Zimbra inside intranet.
Only admin should access this Zimbra OS with older accounts.

How can migrate accounts between them? can i backup these accounts and restore them without issues?

Let me understand this - you are using NE? You cannot have multi server setup with OS edition. If you intend on recycling the licences, then it is not really going to work. If not, then why not install another zimbra NE mailstore and use the migrate mailbox facility to do this. You could even just use a virtual machine as the archive mailstore.
Then when the user accesses the account again, migrate them back.

Ok. Sorry, I was under the impression that you could not have a multi server setup on OSE.
Ok but I am not sure what he means by cannot disable them without wasting license slots. If I am not mistaken, I think he means he does not want to utilize a license for accounts that are not accessed. This is something that will not be able to work...